shrinking percentage of girls at our school

I teach senior English and debate at a small, private (not for profit) high school. Over the last few years we have see a large percentage of our female students leaving the school to attend elsewhere. They have exit interviews with their counselor and have seldom indicated any problems - just a desire to go elsewhere. The situation has gotten so bad that we now have less than 25% female students. We are seeing more fights among the boys, more truancy, and more behavioral problems in the classroom. My question - does anyone know of any studies with regards to the percentage of gender in class and student behavior? Has anyone else had any similar experiences?